Neutral density filters to be employed in the visible (VIS) region of the spectrum are commonly constructed of absorbing filter glasses. Such materials are not useful in the ultraviolet (UV) spectral region between 200 and 350 nm, however, since they have strong absorption bands at these wavelengths.
Neutral density filters for use in the UV region are generally metal or metal alloy films on quartz substrates. INCONEL 600 (TM) is the metallic alloy typically selected for such films, because it closely approximates ideal neutrality. Other materials can also serve the same purpose.
A simple film of metal on a transparent substrate functions as an optical filter throughout the UV and VIS spectral ranges, but attenuates light by reflection as well as by absorption, the amount of reflection typically being 4% to 40% from each surface. When two or more such optical filters are placed in series to effect a higher value of optical density, reflections between them cause the resulting optical density to be significantly different from that predicted from Beer's Law. It is difficult to calculate the correct resultant optical density without a detailed knowledge of the reflectance of each surface. Such knowledge is seldom available.
An additional difficulty with simple metallic films is that they generally carry no protective coatings, and thus are not spectrally stable since the outer metal layer oxidizes and some of this oxide is removed upon cleaning.
The combination of high reflectivity and low spectral stability inherent in simple thin metallic film optical filters reduces their utility for calibrations or other critical work substantially.
Placing a layer of a dielectric material at the surface of the metal film can provide antireflection properties as well as physical protection and resultant spectral stability. However, such a layer does not necessarily reduce the reflectivity of the filter over the wavelength range of interest sufficiently to permit the filter to be used in critical and/or multiple-filter applications. In addition, this approach generally distorts the neutrality of the filter even at transmission values as high as 60%. For filters of lower transmittance the spectral neutrality of the filter is degraded substantially and such an antireflective coating is effective over only a relatively small wavelength interval.
In the paragraphs below, a number of structures having light-filtering properties are discussed to illustrate the state of the art. Not all of these devices are optical filters, as the films were designed for other purposes.
U.S. Pat. No. 3,781,089 of Fay and Cicotta discloses a neutral density filter with reduced surface reflection, for use in the visible portion of the spectrum with photographic apparatus. The filter is formed of alternating layers of a metal or metal alloy and a dielectric on a transparent substrate. The initial and final layers are of the metal or metal alloy. The dielectric layer is stated to have a lower index of refraction than the metal or metal alloy layer, and is preferably silicon monoxide, while the metal is preferably INCONEL(TM). This neutral density filter contains at least five layers, the number of metal or metal alloy layers being one more than the number of dielectric layers. The thickness of the layers is selected to achieve a predetermined optical density, and the thickness of each of the layers of the dielectric is selected to reduce the reflectance of the preceding layers of the filter to a minimum.
The Fay and Cicotta filter design is suitable for use in the visible region of the spectrum, but is unsuitable for critical use in the ultraviolet region since its reflectivity is too high and varies substantially as a function of wavelength. This neutral density filter also is not expected to be spectrally stable since it possesses no protective outer layer. Furthermore, as the Fay and Cicotta patent requires a minimum of three layers of metallic material, this design is appropriate only for filters operating at or above certain optical densities. The first metal layer is of a thickness to produce an optical density of approximately 0.25, and the additional metal layers raise this number. The highest percent transmittance achievable with this design is thus about 55%.
U.S. Pat. No. 3,990,784 of Gelber discloses an architectural glass coated with two layers of metal separated by a layer of a dielectric material. The ratio of the thicknesses of the two metal layers is a constant, while the thicknesses of the metal layers are adjusted to control the transmission properties of the coating. The thickness of the dielectric layer is such that the reflection from the coating is not strongly colored in the visible range. The coating is provided with an antireflection surface, located either adjacent to the substrate or on the outermost layer, facing away from the substrate. An additional layer of dielectric material can be provided to introduce color as seen from the exterior of the architectural glass without changing the low reflection from the architectural glass on the inside of the building. In the event that the multilayer coating is on the outside of the glass, this additional layer of dielectric is located on the outer surface of the glass coating. As shown in FIG. 4 of the reference, when the multilayer coating is to be employed on the inward-facing side of the glass, two additional layers of dielectric material are employed, one on the inward-facing surface of the coating and another between the coating and the glass substrate, this latter material being for the purpose of determining the color as seen from the outside. The inward-facing layer of dielectric provides antireflection properties. The first and last layers of dielectric in the coating are different materials, the former having a high index of refraction and the latter having a low index of refraction. The dielectric spacer layer between the two metal layers also has a high index of refraction, for best color as seen from the outside.
This structure will not function as a low reflection broad band neutral density filter since both its transmittance and its reflectivity vary substantially as a function of wavelength, and the reflectivity is objectionably high at certain wavelengths in the UV and in the visible ranges.
U.S. Pat. No. 4,101,200 of Daxinger discloses a light transmitting and absorbing multilayer coating for a transparent substrate, the initial layer which contacts the substrate being of silicon dioxide, and the remaining layers alternating between chrome and silicon dioxide. This composition is spectrally unstable since the outer layer is a chromium layer which is subject to oxidation and loss of material upon cleaning. Additionally, transmission in the UV varies strongly as a function of wavelength, making it unsuitable as a broad-band neutral density filter.
In view of the limited useful wavelength ranges and limited spectral stabilities of prior art neutral density filters, it would be very desirable to have broad range neutral density optical filters covering a wide range of optical densities, having variations in nominal transmittance no greater than approximately 5% and reflectances no greater than approximately 5% over a substantial portion of the combined UV and VIS spectral range, and having protective outer layers to provide good spectral stability. Such filters are the subject of the present application.